Studies on the Nature of the Centromeric Nucleosome

着丝粒是一种特殊的核小体，作为有丝分裂纺锤体的附着点，在细胞分裂中起着重要作用。它们被认为是标准的八聚体组蛋白复合物，其中一种组蛋白H3被着丝粒特异性变体cenP-A（或cenH3）所取代。我们最近的研究和其他人的研究都支持这样的假设，即至少在细胞周期的某些部分，着丝粒采用四聚体形式。其他研究指出，在细胞周期中可能存在着丝粒重塑，这些发现结合起来，激发了我们对其组成在细胞周期不同阶段可能发生的变化的研究。

Centromeres are specialized nucleosomes playing an instrumental role in cell division as mitotic spindle attachment points. They have been assumed to exist as the standard octameric histone complex with one of the histones, H3, replaced by the centromere-specific variant cenP-A (or cenH3). Our recent studies and others' support the hypothesis that, at least during some part of the cell cycle, the centromere adopts a tetrameric form. Other studies point to possible centromere remodeling during the cell cycle and those findings, combined, motivate our investigation into possible variations in its composition at different stages of the cell cycle. For that purpose, centromeres are isolated from synchronized cells by immuno-precipitation and examined using biochemical and molecular biology tools. In addition we use atomic force microscopy to visualize and compare, at single-molecule scale, centromeres for their shapes, heights and volumes across the cell cycle. The results demonstrate that the centromere is a rather dynamic structure alternating between tetramer and octamer during the cycle. In particular, the centromere remains a tetramer for most of the cycle but changes its composition during the S-phase when DNA is replicated. The transformation probably begins during the transition from G1 to S-phase where we start seeing a mix of tetramers and octamers. The mechanism of such transformation is unknown, but the findings reveal novel processes that will help to further our understanding of the mechanics guiding the cell cycle.